Shrink labels generally fall into two categories: roll-on shrink-on (ROSO) labels and sleeve-type labels (sleeve labels). ROSO labels are film sheets that wrap around a container. Sleeve labels are tubular in configuration and fit around a container by placement over the container. Application of heat to a shrink label that is around a container causes the label to shrink and conform to the container.
To conform to a container, each type of label must shrink preferentially (that is, to a greater extent in one direction than in another direction) in the direction extending circumferentially around the container. ROSO films reside on a container with the machine direction (MD) of the film extending circumferentially around the container. Hence, ROSO films primarily shrink in the film's machine direction (MD) due to preferential machine direction orientation (MDO). In contrast, sleeve labels typically reside on a container with the label's transverse direction (TD) (also known as the cross direction or “CD”) extending circumferentially around the container. Hence, sleeve labels shrink primarily in the film's transverse direction (TD) due to preferential transverse direction orientation (TDO).
ROSO labels are particularly desirably over sleeve labels because they entail less processing and are less costly to produce. ROSO labels are typically in roll form resulting from printing onto an oriented film in a continuous web process. In contrast, sleeve labels, while also available in roll form, require printing, cutting and gluing into sleeves prior to rolling into roll form, complicating the manufacturing process and increasing manufacturing costs for sleeve labels relative to ROSO labels. Furthermore, orienting films in the TD for sleeve labels tends to be more expensive due to the relatively higher cost of the equipment than orienting films in the MD for ROSO labels. Additionally, ROSO application of ROSO labels to containers is typically a faster process than application of sleeve labels.
While ROSO labels offer advantages in production speed, sleeve labels historically have enjoyed an advantage in extent of shrinkage around a container. Sleeve labels typically shrink up to 70 percent (%) around the circumference of a container. In contrast, typical ROSO films historically demonstrate only up to about 20 percent shrinkage around the circumference of a container. For ROSO labels, it is desired that the shrink in the MD be greater than 25 percent, more preferably greater than about 50 percent. Lower shrinkage in the typical ROSO labels is mainly due to: (1) predominant use of oriented polypropylene (OPP), a crystalline polymer, for the film and (2) a limitation on the stress allowed on a glue seam holding the label in place (wrapped) around a container—too much stress on the glue seam can cause the label to shift on the container or, in an extreme case, cause the glue joint to fail and the label to unwrap from around the container. Hence the shrink tension observed for a film is an important factor for determining whether it is suitable for use as a ROSO label, with lower shrink tension being generally preferred. Sleeve labels, which either have no glue joint or have a glue joint that is extensively cured prior to application to a container, can tolerate a greater extent of stress during shrinkage.
Sleeve labels historically enjoy more extensive shrinkage and therefore have conformed better to contoured containers than ROSO labels. However, in view of the production advantages of ROSO labels, it is desirable to identify an oriented film suitable for preparing a ROSO label that can shrink circumferentially around a container to a greater extent than current ROSO labels (that is, more comparable to sleeve labels) but without the detriment of failure at the glue joint of the label.
Polystyrene (PS) is sometimes used to make films for use in shrink labels as PS retains a higher surface energy after corona treatment (necessary to render the surface of a polymer film suitable for printing) for extended periods of time relative to PP. Therefore, unlike PP films, corona treatment of PS films can occur during manufacture rather than just prior to printing into labels.
Polystyrene-based shrink label films often include a high impact polystyrene (HIPS) component in order to improve label toughness (for example, tear resistance). However, rubber particles in a typical HIPS range have an average particle size of greater than one micrometer (see, for example, U.S. Pat. No. 6,897,260, column 4, lines 26-27). Large rubber particles tend to decrease clarity of a label film, interfering with the use of the film for reverse side printing (printing on the side of a label film proximate to the container so that it is readable through the film) as well as with viewing of the container or product through the label. Typical HIPS also contains greater than 7 percent rubber based on total HIPS weight. High concentrations of rubber can hinder the printability of a film, decrease clarity of a film, reduce dimensional stability and undesirably increase gel amount in a final film.
In contrast to copolyester and polyvinyl chloride (PVC) films, use of PS films facilitate bottle and label recyclability, as the lower density allows the label to be more easily separated from the higher density (for example, polyester) bottles. It would be even more beneficial for recyclability if the density of the shrink film were less than 1.0 g/cc as this would allow flotation separation techniques.
Furthermore, lower density films advantageously provide a higher film yield, or more area/lb. of film. Higher density label stock, such as copolyester or PVC films, do not provide similar advantages.
It is desirable to have an oriented multi layer film that is suitable for ROSO or shrink sleeve label applications.
U.S. Provisional application 60/703,385 filed on Jul. 28, 2005, and hereby incorporated by reference in its entirety, teaches a biaxially oriented polystyrene-based film suitable for use as a ROSO label which contained HIPS with a rubber particle size and rubber concentration below that of typical HIPS. It would be desirable to provide a shrink film exhibiting even less shrink tension than the films taught in 60/703,385. It would also be desirable if the overall density of the shrink film was less than about 1 g/cm3 to facilitate separation of the label from the bottle using flotation techniques in the recycling process.
Accordingly, in a first aspect, the present invention is an oriented multi-layer shrink film comprising at least one skin layer comprising an amorphous glassy polymer such as a polystyrene material and at least one core layer comprising a polyolefin layer. The multilayer film can be preferentially oriented in the machine direction, for example using a set of MDO rolls, or in the cross direction, for example using the tenter frame process. The amorphous glassy polymer materials are preferably polystyrene materials which may be general purpose polystyrene (GPPS) or may include other styrenic materials to impart durability and toughness as is generally known in the art. These other materials include materials such as high impact polystyrene, styrenic block copolymers such as K-resin (from Chevron Phillips), SBS (styrene butadiene styrene triblock copolymer), SIS (styrene isoprene styrene), SEBS (styrene ethylene-butylene styrene triblock copolymers), and/or combinations thereof. When high impact polystyrene is desired, it is preferred for the film to contain a high impact polystyrene of a type that has smaller rubber particles and lower rubber concentrations than that of typical HIPS in order to achieve film toughening without hindering printability, clarity or contact clarity of the film. The polyolefin used in the at least one core layer preferably can be selected to control the shrink tension of the film and shrinkage of the film depending on the requirements of the intended application.
In another aspect, the multilayer films of the present invention also comprise one or more tie layers between the polyolefin layer and the polystyrene skin layer.
In another aspect, the present invention is a shrink label comprising a multi-layer film of the first or second aspects wherein the film has printing on one or both sides.